Ln3T2N6 (Ln=La, Ce, Pr; T=Ta, Nb), a New Family of Ternary Nitrides Isotypic to a High Tc Cuprate Superconductor

Dielectric and ultrasound measurements of berthierite FeSb2S4 reported in the literature revealed a phase transition at Tc=348 K. In the present study the phase transition was confirmed by M?ssbauer spectra exhibiting anomalous behavior close to 350 K. Precise lattice parameters measured in the temperature region 305–380 K show a change of slope at about 350 K. The crystal structure of FeSb2S4 has been determined using single-crystal X-ray diffraction data at both room temperature and 370 K above the phase transition. The crystal structure is orthorhombic, Z=4. At 295 K a=11.412(2) ?, b=3.763(1) ?, c= 14.161(3) ?, and R=0.0280, and at 370 K a=11.421(2) ?, b=3.771(1) ?, c=14.173(3) ?, and R=0.0274. No symmetry change has been observed. At both temperatures the space group of FeSb2S4 is Pnma with small changes in atomic parameters and interatomic distances. The transition observed at 348 K is, therefore, to be classified as a weak phase transition going on without essential changes in the crystal structure.